Role of diatomic hydrogen in the electronic structure of ZnO

According to the traditional point of view, interstitial H in ZnO is a shallow donor. We report that the feature can be inhibited by another interstitial H. Although there is a chemical bonding between the two H atoms, they cannot necessarily be considered as a H-2 Molecule: one of the H atoms is tightly bonded into the nearest 0 and forms the O-H complex, the other H atom forms a chemical bond with the nearest Zn and these bonds are stronger than the H-H bond. The defect molecule can be called a H-2* complex and it forms a deep energy level in the band gap. At Fermi levels closer to the conduction-band minimum, the complex is found to be more stable than the two distant H atoms. We discuss the role of H-2* in electronic structure, optical, and electrical properties of ZnO. In particular, formation of the H-2* complex provides a natural limitation for the n-type doping of ZnO by H.